High temperature solid state electrochemical cells are being developed for pulse power applications using for example, highly conducting solid solutions of lithium germanium oxide (Li.sub.4 GeO.sub.4) and lithium vanadium oxide (Li.sub.3 VO.sub.4) as solid electrolytes. The solid solution that can be represented by the formula Li.sub.3.6 Ge.sub.0.6 V.sub.0.4 O.sub.4 has a conductivity of about 0.08 S/cm at 300.degree. C. and has been suggested as a lithium ion conducting solid electrolyte for solid state cells. However, a knowledge of the electrochemical stability range of these electrolytes, that is, potential limits between which the solid electrolyte is stable and does not undergo any electrochemical reduction or oxidation is required prior to their use as solid electrolytes in solid state cells.